<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <script src="../js/vue.js"></script>
</head>

<body>
  <div id="app">
    a:<input type="text" v-model.number="a"><br>
    b:<input type="text" v-model.number="b"><br>
    <output>{{total}}</output>
  </div>
  <script>
    let vm = new Vue({
      el: "#app",
      data: {
        a: null,
        b: null
      },
      computed: {
        total() {
          return this.a + this.b
        }
      },
      methods: {

      },
      created() {

      }
    })
  </script>
</body>

</html>